This invention relates to the use of single channel Neuromuscular Electrical Stimulation (NMESS ) of the lower extremity for the prevention of Deep Vein Thrombosis (DVT) and Pulmonary Embolism (PE) by reducing the pooling of blood in the soleal veins of the calf.
Venous thromboembolic disease (VTED) continues to be a cause of significant morbidity and mortality for individuals immobilized during prolonged travel, after orthopedic surgery, neurologic disorders, and a variety of other conditions.
Virchow in 1845, postulated that changes in blood flow, vessel wall, and blood constituents were responsible for venous thrombosis.1 Studies have shown that at least two of the three factors are nee de d to trigger thrombosis. Reduction of blood flow, especially in the venous sinuses of the calf muscles has long been recognized as an important risk factor.2,3,4 The venous pooling triggers coagulation and at the same time consumes local anticoagulants. This explains the high risk of DVT and PE in spinal cord injury, stroke, and post-surgery where immobility of the lower limbs occurs.
Homans5 observed in 1954 that xe2x80x9cprolonged dependency stasis, a state imposed by airplane flights, automobiles trips and even attendance at the theater, is able unpredictably, to bring on thrombosis . . . xe2x80x9d Until recently it was not known if only travel, without other risk factors, was sufficient to cause venous thrombosis. In 1977, Symington6 showed that trips as short as three to four hours can induce deep vein thrombosis (DVT) and pulmonary embolism (PE), although longer trips are more usual. In 1999, Ferrari7 demonstrated that, in people over 50 years of age, a history of recent travel is a risk factor by itself for thromboembolic disease. He further confirmed that travel longer than four hours increased the risk by four times, even in healthy individuals over 50 years of age. With the decreased legroom in airliners and the escalating time spent in the cramped situation of economy class, Cruickshank coined the term Economy Class Syndrome for the increasing travel-associated venous thromboembolic disease (VTED).8 
In travel associated VTED additional risk factors include pressure on the calves from the back of a seat exacerbating venous stasis,9 dehydration,10 hemocentration,11 and a decrease in fibrinolysis.12 
Current approaches to prophylaxis include mechanical compression using pneumatic compression devices, and anticoagulation therapy. While both have demonstrated effectiveness, they have problems as well. Pneumatic compression equipment is clearly too cumbersome for mobile patients, or during prolonged travel. In addition, AC current is required for these devices and is not practical for these conditions. No manufacturer has designed a battery-operated device. Even if this were done, the battery would be too large and heavy to provide the needed electric current for more the six hours.
Anticoagulation therapy carries the risk of bleeding complications and must be started several days in advance to be effective.
Electric stimulation has theoretical advantages in that it can be started at the time prophylaxis is needed, and can be portable using DC current sources. Previous studies have shown that electrical stimulation is an effective means of increasing venous blood flow and preventing DVT. Nicolaides et al13 studied 116 patients undergoing different operations under general anesthesia. An AC-operated Thrombophylactor supplied single impulse in square waveform. The intensity was strong enough to produce xe2x80x9cbrisk plantar flexion of the foot without violent movement of the legxe2x80x9d. This stimulation was discontinued at the end of the operation just before the patient woke up. It was found that a resting period less than 4 seconds did not allow the soleal veins to fill completely before the next stimulus and resulted in a progressive reduction in stroke volume from the calf muscles. On the other hand, a resting period longer than 5 seconds allowed the soleal veins to fill in the interval between stimuli, resulting in the maximum stroke volume. The optimum rate of stimulation was found to be 12-15 per minute and optimal duration of 50 milliseconds. This stimulus parameter produced the greatest improvement in femoral venous Doppler blood flow. It was shown by a 92% relative reduction in DVT incidence as determined by 125I-fibrinogen testing.
Lindstrom et al14 used groups of stimuli to produce a short-lasting tetanus vs. single stimulus on ten patients being operated on for various abdominal diseases. The aim of the study was to investigate whether a summation of contractions by grouped impulses was more efficient than a single impulse in reducing venous stasis. All patients were anaesthetized during the study. A Whitney strain gauge plethysmograph was used to record changes in calf volume during stimulation. Two surface electrodes were attached just below the knee joint and above the ankle on the back of the leg. A stimulator was used to deliver a square waveform, which could be varied in duration, amplitude and frequency. It was found that this short tetanus reduced calf venous volume three times more effectively than single stimulus. The strength of individual impulses was of the order of 40-50 mA. Using a stimulus duration of 50 milliseconds, they found that grouped stimuli at 8/min, 6 impulses/group and 8 impulses/second were most efficient at reducing venous volume as recorded by plethysmograph. In this study, a lower frequency of 8 grouped impulses per minute was used instead of 12-15 single impulses per minute as advocated in the Nicolaides study.
Merli et al15 performed a prospective study of DVT prophylaxis in acute spinal cord injury (SCI) patients. In the study 48 patients were randomly assigned to saline placebo, subcutaneous low dose heparin or heparin with ES for 23 hours per day over 28 days. Stimulus parameters included a frequency of 10 Hz, duration of 50 microseconds, cycle of 4 seconds xe2x80x9conxe2x80x9d and 8 seconds xe2x80x9coffxe2x80x9d. Both tibialis anterior and gastrocnemius muscles were stimulated to produce a moderately strong contraction. Surveillance for DVT was evaluated by daily 125-I fibrinogen scanning. A significant decrease in the incidence of DVT was noted in the subcutaneous heparin with ES group, but not in the group with placebo or heparin alone.
Jaweed et al16 studied the effects of ES in enhancing femoral venous flow in six normal subjects. A square wave with stimulus duration of 200 microseconds was given at frequencies of 2, 5 and 10 Hz in 2 to 3 sessions over four weeks. Maximum peak velocity was measured bilaterally in the supine posture. It was found that electrical stimulation (ES) at 10 Hz significantly increased femoral venous flow but not at lower frequencies of 2 and 5 Hz.
There a number of U.S. patents that teach methods of applying electrical stimulation for the prevention of DVT. These include the following patents: the Powell, III patent, U.S. Pat. No. 5,358,513; the Tumey patent, U.S. Pat. No. 5,674,262; the Dennis, III patent, U.S. Pat. No. 5,782,893; the Katz patent, U.S. Pat. No. 5,643,331; and the more recent Katz patent, U.S. Pat. No. 6,002,965. What these patents have in common is that they all describe methods of electrically stimulating the calf muscle.
These methods do enhance blood flow by causing the calf muscle to contract. But they also have some drawbacks that the present invention overcomes.
Positioning the electrodes on the calf muscle can be problematic. The differences in size and shape of peoples calf muscles requires fitting cuffs to ensure accurate placement of the electrodes. Often these electrode presenting cuffs or bands shift or slide down the leg with use. One irritating problem is the removal of the electrode from the calf when the hair on the leg becomes bound to the sticky electrode
Another difficulty with these calf stimulators is that the fatty tissue layer has a relatively high resistance to electric current. This fatty surface layer, between the electrode and the muscle beneath, requires higher current settings in order to adequately stimulate the muscle to contraction. These higher current settings, especially with obese patients, can cause significant discomfort to the skin area around the electrode.
The size of the calf muscle also requires a relatively high current setting to cause it to contract. Again, higher current pulses are more likely to cause sensations that the user finds unpleasant, especially when administered over a long period of time.
What is need therefore is method of enhancing blood flow in the lower leg without the problems associated with stimulating the calf muscle.
What is also needed is a device that can be put on effortlessly, without fitting or adjustments, that doesn""t shift or slip out of place during use.
What is also required is a device that is not uncomfortable to use, and is in fact pleasurable to use.
What is also needed is a device that can be used by obese patients who are at greatest risk for acquiring DVT.
A significant number of DVT cases occur on extended airplane flights where having a pleasurable experience, convenience of application, speed of fitting and removing, are all essential to their wide-scale adoption and use.
The present invention comprises a method and a device for preventing DVT by electrically stimulating the foot muscles by applying one electrode over or proximal to the heel and the other over the intrinsic muscles on the plantar surface of the foot, or proximal to them, for example on or around the ball of the foot.
This method and the preferred embodiments of the invention, described herein, have the advantage of: locating the electrodes where there is little or no body hair to become entangled in the sticky electrodes; locating the electrodes where there is little fat tissue, even in obese users; being applied by simply putting on an article of footwear; requiring only a few different sizes for virtually all users; being placed in close proximity to the muscle that is being stimulated; and being a pleasure to use. Because the physiology of the foot is relatively uniform among the population, compared to marked differences in calf size and fatty content, the method has the additional advantage in that the NMES unit that delivers the electrical current can be made to have a relatively narrow range of intensities to ensure that users will not inadvertently use a setting that would otherwise cause them pain.
The fact that the electrical stimulation of the foot would have the effect of enhancing blood flow in the calf and thus help prevent deep vein thrombosis and related maladies is quite unexpected. For example, the Tumey patent, U.S. Pat. No. 5,674,262, referred to above, coupled electrical stimulation of the calf muscle with pneumatic compression of the foot, rather than electric stimulation of the foot. It has not heretofore been understood that by electrically stimulating such a small group of muscles that blood flow could be enhanced thereby reducing the pooling of the blood in the soleal veins of the calf.
Two of the inventors, Kaplan and Czyrny, performed a trial study on two normal subjects and one obese subject between the ages of 50 to 80. The subjects were seated in chairs placed at a fixed distance apart. One lower limb was randomly selected for stimulation, therefore each subject served as his own control. Unlike previous studies where electric stimulation was applied to the calf muscles, surface electrodes were placed over the intrinsic muscles on the plantar surface of the foot, or proximal to them, for example on or around the ball of the foot, and over or proximal to the heel. Also, compared to those previous studies, a much lower intensity of stimulation was used.
For the method that is the subject of this invention, the intensity of the electrical stimulation required is only that necessary to create a slight visible muscle twitch of the foot muscles, or to put it another way a minimally visible or palpable muscle contraction. Previous studies used stimulus intensities much greater such that a vigorous muscle contraction was produced.
Kaplan and Czyrny measured Popliteal (calf) venous blood flow bilaterally using a Doppler ultrasound device at 0, 15, 120 and 240 minutes after stimulation began. The Doppler ultrasound evaluator was blinded as to the limb stimulated. Patients were also asked to fill out a brief questionnaire regarding their acceptance and tolerance of the electrical stimulation. The results of those tests demonstrated a significant increase in popliteal (calf) venous blood flow velocity across the 4-hour stimulation period for the stimulated leg during the majority of time frames measured. This is consistent with reduced venous pooling in the soleal veins of the calf It should be noted that the soleal veins refer to the plexus of veins located within the calf muscles. The term is not related to the sole of the foot.
Review of the patient questionnaires revealed excellent acceptance and tolerance of the electrical stimulation that was used in the study. The degree of tolerance was significantly greater than a previous study Kaplan and Czyrny performed looking at the tolerance of electrical stimulation of the calf muscles. Therefore it is believe that a mild electrical stimulation of the foot muscles is an effective, and well tolerated method of enhancing popliteal venous blood flow. Further studies will be required to fully elucidate the method and its application to various types of users and extended periods of use.
The articles listed herewith represent literature relating to electrical stimulation of leg muscles to inhibit deep venous thrombosis.
1. Virchow R. Ein Vortrag Uber die Throm bose vom Jahre 1845. In: Virchow R, ed. Gesammelte Abhandlungen Zur weissenschaftlichen Medizin. Frankfurt: Meidinger, 1845: 478-86.
2. Simpson K, Shelter. Death from pulmonary embolism. Lancet 1940; ii: 744.
3. Ledermann J A, Keshavarzian A. Acute pulmonary embolism following air travel. Postgrad Med J 1983; 39: 348-53.
4. Mammen E F. Pathogenesis of venous thrombosis. Chest 1992; 102(suppl): 640s-644s.
5. Homans J. Thrombosis of deep leg vein due to prolonged sitting. N England J Medicine 1954; 250: 148-9.
6. Symington I A, Stack B H R. Pulmonary thromboembolism after travel. Br J Dis Chest 1977,71: 138-40.
7. Ferrari E, Chevallier T, Chapelier A, Baudouy M. Travel as a risk factor for venous thromboembolic disease: a case-control study. Chest 1999; 115:440-444.
8. J Cruickshank, R. Gorlin, B. Jennett. Air travel and thrombotic episodes: the economy class syndrome. Lancet, August 1988.
9. Thomas J E P, Abson C P, Caims N J W, Pulmonary embolism. A hazard of air travel. Cent Air J Med 1981; 27: 85-87.
10. Simon R, Krol J. Jet xe2x80x9clegxe2x80x9d, pulmonary embolism and hypoxia (letter). Lancet 1996; 348: 416.
11. Moyses C. Economy class syndrome (letter). Lancet 1988; 1: 1077.
12. Gertler J P, Perry L, L""Italien G, et al. Ambient oxygen tension modulates endothelial fibrinolysis. J Vas Surg 1993; 18:939-946.
13. Nicolaides A N, Kakkar V V, Field E S, et al. Optimal electric stimulus for prevention of deep vein thrombosis. Br M J 1972; 3: 756-758.
14. Lindstrom B, Korsan B K, Jonsson, et al. Br J Surg 1982; 69: 203-206.
15. Merli G J, Herbison G J, Ditunno, et al. Deep vein thrombosis: prophylaxis in acute spinal cord injured patients. Arch Phys Med Rehab 1988, 69:661-664.
16. Jaweed M M, Herbison G J, Merli G, et al. Enhancement of venous blood flow velocity by low frequency electrical stimulation in normal human subjects. Arch
17. Phys Med Rehab 1986; 67: 61 (abstract)
The invention is comprised of a method for preventing DVT, ankle edema and venostasis and a device that includes a single channel sequential neuromuscular electrical stimulation (NMES) unit. The NMES unit 10 can be any NMES unit that is battery powered, compact and can be programmed to deliver the stimulus profile described below or such other profile that is found to be efficacious, such as the Focus(trademark) manufactured by Empi Inc., 599 Cardigan Road St. Paul, Minn., U.S.A. In order to simplify the patient""s ability to properly apply the NMES device, the stimulator generates biphasic symmetrical square wave pulses with stimulus parameters that our study demonstrated to result in optimum venous blood flow. The stimulus frequency is fixed at 50 pulses per second, the stimulus duration is set at 300 microseconds, the ramp up time at 2 seconds, the ramp down time at 2 seconds, and the stimulus cycle set at 12 seconds on and 48 seconds off. Once set in advance by the Doctor, manufacturer or user, the only adjustment necessary on the part of the patient is a stimulus intensity dial. This allows for a current up to 20 milliamperes to be delivered. The user adjusts the intensity to the point needed to produce a minimally visible or palpable muscle contraction. The output leads of the stimulator are attached through a conductor to electrodes of various types including, self-adherent surface electrodes. These electrodes being of opposite polarity and creating an electrical potential difference between themselves and the tissue that separates them.
While the type of electrical pulse generating unit and those characteristics and routine for administering the pulses described above have been found to be very effective in increasing blood flow, it is to be understood that any pulse generator that causes the foot muscle to periodically and gently contract, such that the user does not experience excessive pain, is within the ambit of the invention herein disclosed.
The unique feature of the invention is the use of a pre-programmed single channel stimulator that delivers a very mild, well-tolerated intensity that results in a mild contraction of the foot muscles. This is accomplished by only stimulating the soles of the feet. The surface electrodes were placed over intrinsic muscles on the plantar surface of the foot, or proximal to them, for example on or around the ball of the foot, and over or proximal to the heel. By stimulating in this manner, blood pooling in the calf veins is prevented. Preliminary studies by two of the inventors, Kaplan and Czyrny, using Doppler ultrasound techniques have shown that this method may be as effective as stimulation of the calf muscle group en masse as other devices do, and significantly better tolerated than these devices.
But in order for the method to work effectively and be self-administered without supervision and with minimal instructions, the placement of the electrodes and the type of electrodes used are critical. Elderly patients, who often have impaired abilities to self-administer treatments are the very ones that would benefit most the method that is herein described. The means for positioning the electrodes on the foot must therefore be convenient and effective.
The preferred embodiments of the invention include footwear and accessories to footwear that incorporated NMES devices and electrodes that can be used with or without socks and stockings. Some of the preferred embodiments lend themselves to one time use, such as would be the case for those used in airplane travel.